1. Field of the Invention
This invention relates to a process for the production of tablet form detergent compositions, more particularly for use in dishwashing machines.
2. Statement of the Prior Art
Powder form, granular or tablet form detergent compositions for crockery in dishwashing machines consist essentially of sodium tripolyphosphate, alkali silicate, generally alkali metasilicate, and optionally of a low-foam, preferably chlorine stable, nonionic surfactant and also bleaching agents, preferably of the type which give off chlorine in aqueous solution. Their primary function is to facilitate the removal of stains left by tea, coffee or fruit juices.
The compositions are strongly alkaline because of their high content of alkali silicates and sodium tripolyphosphate. The solutions in which they are used have a pH value of from about 11 to 12. The compositions do not contain any additions of sodium hydroxide because, in view of the contact times normally necessary in domestic dishwashing machines, the excessive alkalinity would attack porcelain glazed on decorations and glasses. This aggressive effect is known as corrosion. By contrast, silicate synergistically supports the cleansing power of the tripolyphosphate and, in certain concentrations in the tripolyphosphate/silicate mixture, inhibits corrosion.
The known comspositions are generally produced by standard methods, such as mixing or granulation. They are used in powder form or granulate form, again in standard concentrations, i.e. in quantities of from about 30 to 50 g (about 1 to 1.75 oz.) per machine load.
Powder form or granular compositions of this type are described, for example, in German Patent Application Nos. 23 50 295 (corresponding to British No. 1,442,885); 23 58 249 (corresponding to U.S. Pat. Nos. 3,816,320 and 3,936,386); 29 13 145; 24 34 309 (corresponding to U.S. Pat. No. 3,933,670); and 26 02 357 (corresponding to U.S. Pat. No. 4,083,795).
According to French Pat. No. 1,472,680, (corresponding to U.S. Pat. No. 3,390,092) circular tablets, for example having a diameter of 3 cm, a thickness of 1.2 cm and a weight of about 12 g (from which a density of more than 1.4 g/cc can be calculated) may be produced from simple mixtures of known detergent ingredients in conjunction with a tableting aid using pressures of from about 70 to 1000 kg/cm.sup.2. About four of these tablets give substantially the same result as a powder form detergent filling of 50 g per machine load.
Tablets of the type mentioned above are also described in German patent application No. 28 57 001 (corresponding to U.S. Pat. No. 4,219,436). They are also obtained by compressing simple mixtures of the constituents under appropriate pressures. In this case, alkalinity is determined not through the pH value, but instead by ascertaining how much N-hydrochloric acid is necessary to reduce the pH value of a solution of 100 g of the detergent composition in water to pH 9 and comparing the quantity of sodium hydroxide dissolved in the same quantity of water which is required for the same quantity of N-hydrochloric acid. The result is expressed as the "alkalinity equivalent". In the case of the known dishwashing compositions, the values are generally above 15 g of NaOH, based on 100 g of detergent composition.
The particular pressure level and the resulting density of the tablets are always determined by the necessary compromise between the strength and dissolving rate of the tablets. Other factors affecting those properties are the nature of the constituents themselves and suitable tableting aids.